The present invention relates to a method for continuously producing the compound cyclic formal, also sometimes known as methylal or dimethoxy methane, which is used as an intermediate in producing solvents and drugs, and as a raw material in production of acetal resin, etc., and related formal compounds such as ethylene glycol formal, diethylene glycol formal and 1,4-butanediol formal, for example. In more detail, the present invention relates to a method for continuously producing cyclic formal having reduced contents of impurities such as unreacted aldehyde or water.
It is known that cyclic alkylene formals such as ethylene glycol formal, diethylene glycol formal and 1,4-butanediol formal can be obtained by cyclic reaction between a corresponding alkylene glycol or alkylene oxide and aldehyde. For example, German Patent No. 191420 discloses a method for producing ethylene glycol formal, which is a typical cyclic formal, by reacting ethylene glycol and formaldehyde with each other in the presence of an acid catalyst.
Furthermore, Japanese Patent Laid-Open (Kokai) No. 49-62469 discloses a method for producing ethylene glycol formal by reacting ethylene glycol and paraformaldehyde in the presence of an acid catalyst.
Moreover, Japanese Patent Laid-Open (Kokai) No. 7-224055 discloses a method for obtaining a cyclic formal by reacting an alkylene glycol and formaldehyde with each other at a specific molar ratio.
Cyclic formals obtained by the above methods are usually obtained as a mixture of a cyclic formal containing a large amount of water, and water. Methods for decreasing water content in the mixture containing water, for obtaining a purified cyclic formal, are also known. For example, Japanese Patent Laid-Open (Kokai) No. 49-62469 discloses a method for producing a purified cyclic formal by adding cyclohexane to the reaction distillate obtained by reacting paraformaldehyde and ethylene glycol with each other in the presence of an acid catalyst, and rectifying the mixture. Japanese Patent Laid-Open (Kokai) No. 3-24071 discloses a purification method in which a crude cyclic acetal is treated with an aqueous alkali solution and/or a solid alkali hydroxide compound. Japanese Patent Laid-Open (Kokai) No. 5-241217 discloses a purification method in which the reaction distillate obtained by reacting formaldehyde and ethylene glycol in the presence of an acid catalyst is brought into contact with ethylene glycol in countercurrent flow in a distillation tower. Japanese Patent Laid-Open (Kokai) No. 7-206716 discloses a purification method in which a mixture containing a cyclic formal and water is brought into contact with a hydrophilic solvent having a boiling point of 180.degree. to 250.degree. C. in a distillation tower. Furthermore, Japanese Patent Laid-Open (Kokai) No. 7-206717 discloses a method in which the purified cyclic formal obtained in Japanese Patent Laid-Open (Kokai) No. 7-206716 is further treated with a dehydrating agent.